Mega Sableye was just revealed at Nintendo’s Kids Conference at E3 and uploaded to Pokemon’s YouTube channel. Some details were also revealed through a press release:
Sableye is a Dark- and Ghost-type Pokémon. Mega Sableye has concentrated all of the energy from Mega Evolution into the jewel which adorns its chest. Once Sableye has Mega-Evolved, this jewel will swell and become detached from its body. Mega Sableye uses the jewel as a powerful defensive tool in battle. From behind the shelter of the jewel, it can readily exploit the opponent’s weaknesses and take advantage of any openings. It is said that the jewel hardens into such strength that no attack can even make a scratch!